Adding on

[fbeatty]

I am planning on adding a double 40 amp breaker on my existing panel in the garage with 10/3 and running it outside to my shed where I am installing a box with two 20 amp breakers which I will run 10/2 to the outlets and lights. Is this a sound move?

[HayZee518]

For a double 40 - the correct wire size is 8/4. Your branch circuits of 20 amp take 12/2 wire with ground.

[kactuskid]

Is this box you plan to install in the shed with the two 20 amp breakers a subpanel installation. If this is a subpanel installation then you need to install a grounding electrode in the ground at the shed. There are also specific rules to follow when installing a subpanel, one important rule is that the ground and neutral buss bars must remain separte.

If this is not a subpanel installation, then you will be installing what is called a multiwire circuit, which would require a double pole 20 amp breaker maximum installed in your garage panel and no breakers are then allowed to be installed at your shed. This circuit shares the neutral wire so there is a 240 volt potential between both hot wires. This can be very dangerous for a DIYer installation as it is imperative that all neutral wires be pigtailed to prevent an open neutral from occurring.

More details are needed, what are your power needs at the shed, what do you plan on running out there? Is 240 volts needed or just 120 volts.